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T4 template for immutable type generation
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README.md

ImmutableObjectGraph

Build status NuGet package Join the chat at https://gitter.im/AArnott/ImmutableObjectGraph

This project offers code generation that makes writing immutable objects much easier. For instance, the following mutable class:

public class Fruit {
    public string Color { get; set; }
    public int SkinThickness { get; set; }
}

Is very short, easily written and maintainable. The equivalent immutable type would require methods offering mutation, and ideally several other support methods and even a Builder class for use in conveniently handling the immutable object when mutation by creating new objects may be required. These codebases for immutable objects can be quite large.

To reduce the burden of writing and maintaining such codebases, this project generates immutable types for you based on a minimal definition of a class that you define.

Supported features

  • Field types may be value or reference types.
  • When field types are collections, immutable collections should be used that support the Builder pattern.
  • When field types refer to other types also defined in the template file, an entire library of immutable classes with members that reference each other can be constructed.
  • Batch property changes can be made with a single allocation using a single invocation of the With method.
  • Builder classes are generated to allow efficient multi-step mutation without producing unnecessary GC pressure.
  • Version across time without breaking changes by adding Create and With method overloads with an easy application of [Generation(2)].

Usage

You can begin using this project by simply installing a NuGet package:

Install-Package ImmutableObjectGraph.Generation -Pre

On any source file that you use the [GenerateImmutable] attribute in, set the Custom Tool property to: MSBuild:GenerateCodeFromAttributes

Example source file

[GenerateImmutable]
partial class Fruit
{
    readonly string color;
    readonly int skinThickness;
}

Example generated code

The following code will be generated automatically for you and added to a source file in your intermediate outputs folder:

partial class Fruit
{
    [System.Diagnostics.DebuggerBrowsableAttribute(System.Diagnostics.DebuggerBrowsableState.Never)]
    private static readonly Fruit DefaultInstance = GetDefaultTemplate();
    private static int lastIdentityProduced;
    [System.Diagnostics.DebuggerBrowsableAttribute(System.Diagnostics.DebuggerBrowsableState.Never)]
    private readonly uint identity;
    protected Fruit(uint identity, System.String color, System.Int32 skinThickness, bool skipValidation)
    {
        this.identity = identity;
        this.color = color;
        this.skinThickness = skinThickness;
        if (!skipValidation)
        {
            this.Validate();
        }
    }

    public string Color
    {
        get
        {
            return this.color;
        }
    }

    public int SkinThickness
    {
        get
        {
            return this.skinThickness;
        }
    }

    internal protected uint Identity
    {
        get
        {
            return this.identity;
        }
    }

    public static Fruit Create(ImmutableObjectGraph.Optional<System.String> color = default(ImmutableObjectGraph.Optional<System.String>), ImmutableObjectGraph.Optional<System.Int32> skinThickness = default(ImmutableObjectGraph.Optional<System.Int32>))
    {
        var identity = ImmutableObjectGraph.Optional.For(NewIdentity());
        return DefaultInstance.WithFactory(color: ImmutableObjectGraph.Optional.For(color.GetValueOrDefault(DefaultInstance.Color)), skinThickness: ImmutableObjectGraph.Optional.For(skinThickness.GetValueOrDefault(DefaultInstance.SkinThickness)), identity: identity);
    }

    public Fruit With(ImmutableObjectGraph.Optional<System.String> color = default(ImmutableObjectGraph.Optional<System.String>), ImmutableObjectGraph.Optional<System.Int32> skinThickness = default(ImmutableObjectGraph.Optional<System.Int32>))
    {
        return (Fruit)this.WithCore(color: color, skinThickness: skinThickness);
    }

    static protected uint NewIdentity()
    {
        return (uint)System.Threading.Interlocked.Increment(ref lastIdentityProduced);
    }

    protected virtual Fruit WithCore(ImmutableObjectGraph.Optional<System.String> color = default(ImmutableObjectGraph.Optional<System.String>), ImmutableObjectGraph.Optional<System.Int32> skinThickness = default(ImmutableObjectGraph.Optional<System.Int32>))
    {
        return this.WithFactory(color: ImmutableObjectGraph.Optional.For(color.GetValueOrDefault(this.Color)), skinThickness: ImmutableObjectGraph.Optional.For(skinThickness.GetValueOrDefault(this.SkinThickness)), identity: ImmutableObjectGraph.Optional.For(this.Identity));
    }

    static partial void CreateDefaultTemplate(ref Template template);
    private static Fruit GetDefaultTemplate()
    {
        var template = new Template();
        CreateDefaultTemplate(ref template);
        return new Fruit(default(uint), template.Color, template.SkinThickness, skipValidation: true);
    }

    partial void Validate();
    private Fruit WithFactory(ImmutableObjectGraph.Optional<System.String> color = default(ImmutableObjectGraph.Optional<System.String>), ImmutableObjectGraph.Optional<System.Int32> skinThickness = default(ImmutableObjectGraph.Optional<System.Int32>), ImmutableObjectGraph.Optional<uint> identity = default(ImmutableObjectGraph.Optional<uint>))
    {
        if ((identity.IsDefined && identity.Value != this.Identity) || (color.IsDefined && color.Value != this.Color) || (skinThickness.IsDefined && skinThickness.Value != this.SkinThickness))
        {
            return new Fruit(identity: identity.GetValueOrDefault(this.Identity), color: color.GetValueOrDefault(this.Color), skinThickness: skinThickness.GetValueOrDefault(this.SkinThickness), skipValidation: false);
        }
        else
        {
            return this;
        }
    }

#pragma warning disable 649 // field initialization is optional in user code

    private struct Template
    {
        internal System.String Color;
        internal System.Int32 SkinThickness;
    }
#pragma warning restore 649
}

The integration of the code generator support in Visual Studio allows for you to conveniently maintain your own code, and on every save or build of that file, the code generator runs and automatically creates or updates the generated partial class.

Known Issues

When defining more than one immutable type, you may need to keep the arguments to the [GenerateImmutable] attribute consistent for every type. The generator currently assumes that every type has the same arguments as every other type and as a result, for example, generating a Builder from one type and referencing another type, that other type will be assumed to also have a Builder even when it does not, leading to compiler errors.

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